Bipolar transistors with raised bases are well-known in the semiconductor transistor art.
U.S. Pat. No. 4,789,643, filed Sept. 16, 1987, shows a device in FIG. 6 thereof which may be characterized as having a raised base. In addition, the reference shows a direct connection between the intrinsic and extrinsic elements of the base. All of the elements of the structure result from the deposition of epitaxial layers of semiconductor material which are then appropriately masked and subjected to ion-implantation steps to form insulating regions and the device emitter. The reference does not use any polycrystalline layers from which base and emitter are formed nor does it incorporate method steps which form polycrystalline regions over polycrystalline material and single crystal regions over single crystal material such that, upon deposition and diffusion of a second layer, a single crystal emitter region with its associated electrical connection and a base-collector p-n junction are formed. There is no indication in the reference that differential diffusion rates in single crystal material and polycrystalline material are utilized.
U.S. Pat. No. 4,499,657, originally filed Feb. 29, 1980, however, does recognize that semiconductor material deposited over insulation will be polycrystalline in character and that single crystal material deposited over single crystal material will remain single crystal. Using this phenomenon, polycrystalline regions form the extrinsic base while single crystal regions form the intrinsic base, collector and emitter. The reference also recognizes that there are differential rates of diffusion in polycrystalline material and single crystal material. In the process of the reference, a single layer having single crystal and polycrystalline regions is deposited. In subsequent steps, two ion implantations are utilized to form the base and emitter regions. In the instance of the latter element, a separate metallization is required. This reference does not recognize that a raised base device may be obtained by conformal deposition of a pair of opposite conductivity type layers containing single crystal and polycrystalline regions and by an out-diffusion step which simultaneously forms the device emitter, the device emitter-base p-n junction, a p-n junction isolated base and an electrical connection to the emitter.
U.S. Pat. No. 4,431,460, filed Mar. 8, 1982, shows a single layer of semiconductor material deposited over a single crystal region of a substrate and over insulation covered polysilicon silicon. The single layer is subjected to ion implantation steps such that upon heating, both n and p-type dopants are introduced into the underlying substrate forming emitter and base within the substrate. There is no use of a second deposited layer nor is there any recognition that single crystal and polycrystalline regions are formed in the only polycrystalline layer deposited. Ultimately, the deposited layer forms part of the emitter contact in the reference.
U.S. Pat. No. 4,504,332, originally filed Sept. 6, 1979, shows a structure wherein the extrinsic and intrinsic bases, the emitter and the collector all stand above the substrate layer. They are all self-aligned. The process taught involves a doped layer of insulation over which a semiconductor layer is formed having a single crystal region over single crystal material and polycrystalline regions over polycrystalline material. By out-diffusing from the doped insulator, opposite conductivity type regions are formed which ultimately become extrinsic bases and a single crystal region remains between the out-diffused extrinsic bases. In subsequent steps, an intrinsic base is ion implanted and an emitter region is formed by diffusion of a dopant from an oxide. Thus, while many of the same phenomena used in the present application are invoked in this reference, such as differential diffusion rates and the formation of polycrystalline and single crystal regions over insulating or single crystal materials, respectively, a rather complex process is used to complete the transistor after a single crystal region is obtained. There is no recognition that by depositing a second opposite conductivity type layer and diffusing that an emitter, a base-emitter p-n junction and an electrical connection to the emitter could be simultaneously formed. Recognition of this step would greatly simplify the process of the reference.
U.S. Pat. No. 3,600,651, filed Dec. 8, 1969, shows a conformally deposited semiconductor layer on insulation and on single crystal semiconductor material providing polycrystalline regions over insulation and a single crystal region over single crystal material. Dopants placed in adjacent insulation provide for introducing dopant into the polycrystalline regions. In all the vertical bipolar devices disclosed in this reference, the emitters are separately diffused into the single crystal region without invoking the greatly simplified step of the present application of using a second layer of opposite conductivity type and diffusing to form the device emitter and its electrical connection.
It is, therefore, an object of the present invention to provide a bipolar transistor in which the base, emitter and electrical connection to the latter are all formed from a pair of conformally deposited semiconductor layers.
Another object is to provide a bipolar transistor having a raised intrinsic base wherein the latter is connected directly to the extrinsic base which is similarly raised.
Still another object is to provide a bipolar transistor in which a portion of the single crystal intrinsic base overlaps the ROX region of the device such that the edge punchthrough or breakdown is substantially eliminated.
Yet another object is to provide a process for fabricating bipolar transistors which is greatly simplified relative to prior art techniques.
Another object is to provide a process for fabricating bipolar transistors wherein succeeding method steps after a first conformal layer deposition do not disturb the results of the preceding steps thereby preventing edge punchthrough.